Originally left advertising below every last selfie in the form of tags (6)
I believe the answer is:
labels
'tags' is the definition.
('label' can be a synonym of 'tag')
'originally left advertising below every last selfie' is the wordplay.
'originally' suggests taking the first letters.
The first letters of 'left advertising below every last selfie' is 'LABELS'.
'in the form of' acts as a link.
(Other definitions for labels that I've seen before include "tags,brands" , "Appends a category to something or someone" , "'Trade names, of designers say (6)'" , "Tags; classifies" , "Present tags" .)